The present invention relates to a method for profiling a fibrous web, advantageously a paper web, in a multiroll calender which includes: at least an upper set or stack of rolls having at least one calendering nip which is formed in a cross-direction of the web between two rolls and which profiles a first side of the web in the cross-direction by regulating distribution of load to the first side in the nip in a cross-machine direction; at least one lower set or stack of rolls having at least one calendering nip which is formed in the cross-direction of the web between two rolls and which profiles a second side of the web opposite to the first side in the cross-direction by regulating distribution of load to the second side in the nip in the cross-machine direction; and a reversing nip which does not profile the web and which is formed between the lowermost roll of the upper set or stack of rolls and the uppermost roll of the lower set or stack of rolls.
In today's multiroll calenders, calendering takes place by the method “from the top downward”, i.e. paper is passed first through a top nip to an upper set of rolls, in which only one of the paper surfaces is calendered. After that, paper is passed through a reversing nip to a lower set of rolls, in which the other surface is treated. Further, to control the CD profile of the caliper and surface properties of the paper, the top and bottom nips are generally provided with a profiling operation in which the distribution of the load applied to the paper is regulated in the cross direction. This kind of prior art arrangement is shown in FIG. 1, which shows a multiroll calender comprising ten rolls. In this calender, the calendering nips profiling the web are the first and the ninth nip. In that connection, calendering takes place according to the principle that the bottom surface of the web is calendered first four times in the set of rolls situated above the reversing nip, said set of rolls comprising hard-surfaced thermo rolls 3 and soft-surfaced backing rolls 2, for example, polymer-covered rolls, after which the web is passed through the reversing nip, which is followed by calendering of the top surface of the web four times in the lower set of rolls situated beneath the reversing nip, said set of rolls comprising hard-surfaced thermo rolls 3 and soft-surfaced backing rolls 2, for example, polymer-covered rolls. Thus, the side of the web to be treated at each particular time is against a thermo roll of the multiroll calender. In the sets of rolls, the transfer of the web from one nip to the next is guided by means of guide rolls 4.
In the conventional construction, the upper and lower parts of the set of rolls thus apply calendering to the different surfaces of the paper. From the point of view of the paper, the situation is, however, very different in the upper and in the lower set of rolls, even though the situation would involve equal loading. In the first nip, paper is uncalendered, porous, moist and, at least in off-line arrangements, cool. In the lowermost nips, paper has already been compressed almost to its final density and, as a result of successive nip contacts, the paper has become considerably warmer (the average temperature has increased by about 40-50° C.) and dried (at least 2-3 percentage units). Thus, treatment of the different sides of the paper is performed in succession and in markedly different conditions.
It is known that in constant conditions under successive compression loads, most of the permanent compression of paper is produced during the first (1-5) times of loading. After that, the paper has already become compressed for the most part and deformations are more elastic, i.e. reversible. In practice, however, the warming of paper increases the amount of plastic compression of paper in the lower set of rolls. On the other hand, it may be thought that compression is higher in the first nips because of the higher moisture content of the paper.
Further, the gradient effects of calendering are more marked in the upper set of rolls because of a large temperature difference created between the thermo roll and the interior of cool paper. By contrast, in the bottom nip where paper has already warmed up by the effect of the preceding nips, it is not possible to achieve a corresponding temperature gradient with the same roll temperature. It may thus be noted that the gradient effects are more distinct in the first nips in the upper set of rolls and the operation of the lower set of rolls is based more on the compression of the paper structure which has been warmed throughout.
In conventional arrangements, profiling that takes place in the top nip operates relatively effectively, whereas it has been observed in practice that in the bottom nip the profiling of the surface properties of paper is rather ineffective, often even insufficient.
The primary aim of the present invention is to remove or at least to reduce the above-mentioned drawbacks, weaknesses associated with conventional calendering, and to provide an improved calendering method in order to assure profiling of the surface properties of both sides of paper in a more effective manner than before and to achieve an increased quality effect and a reduction in the dimensioning and loading values of the profiling devices when profiling becomes easier, which saves roll covers.